1. Field of the Invention
This invention relates to a hydraulic pressure-control valve for an automatic vehicle transmission, more specifically to an improved hydraulic pressure-control to be used for engaging a frictional engaging element such as a start clutch in an automatic vehicle transmission.
2. Description of the Related Art
Automatic vehicle transmission start devices include the start clutch of the continuously variable transmission and the lock-up clutch of the torque converter of the continuously variable/multi-step transmission. As taught by Japanese Patent Laid-Open Application Nos. Sho 62(1987)-238,129 and Hei 2 (1990)-150,554, for example, the start clutch is controlled for smooth vehicle starting (creeping) by regulating the hydraulic pressure (to be supplied) with an electric actuator, specifically, an electromagnetic solenoid valve (clutch control valve).
Other hydraulic pressure-control valves aside from those the prior art referred to above are also known, such the one illustrated in FIG. 22. The illustrated pressure-control valve has a spring 102 and a pressure-regulating valve 104 disposed opposite an electromagnetic linear solenoid 100.
The pressure-regulating valve 104 is biased to the left in the drawing by the spring 102. When the plunger 100a of the linear solenoid 100 is excited, it moves to the right in the drawing against the spring force and presses the pressure-regulating valve 104 to the right.
In this specification, words indicating direction such as left, right, up and down mean direction in the drawings.
Hydraulic pressure from a hydraulic pressure source (not shown) regulated to line pressure CR is supplied through a line-pressure supply line (passage) 106, passes through a gap defined by the position of the pressure-regulating valve 104 and a valve body 108, proceeds to an output line (passage) 110 and is supplied to the hydraulic chamber of a start clutch (not shown).
A feedback line (passage) 112 branching from the output line 110 feeds part of the output pressure back to the rear end of the pressure-regulating valve 104. In the course of hydraulic pressure being supplied to instigate clutch engagement, when the leftward force on the valve owing to the feedback pressure becomes equal to the solenoid load, the gap between the pressure-regulating valve 104 and the valve body 108 is closed to stop the hydraulic pressure increase. The hydraulic pressure (to be supplied to the clutch) thus varies with the increase/decrease in the current through the solenoid (the solenoid load). The portions marked with x's in the drawing are drain ports.
As shown by the characteristic curve in FIG. 23, the hydraulic pressure to be supplied to the clutch by the conventional pressure-control valve shown in FIG. 22 is a linear function of the solenoid current (solenoid load). Therefore, when hydraulic pressure control is effected in the region where the vehicle is creeping or starting or in the region where low torque capacity hydraulic pressure control is effected at the lock-up clutch of the torque converter (i.e., the clutch is slip- controlled), the control is difficult to achieve with good precision owing to the large effect of solenoid hysteresis, as can be seen from the characteristic curve of FIG. 24 showing how hydraulic pressure (to be supplied to the clutch) varies with solenoid current.
Moreover, since the change in hydraulic pressure per unit load is the same both at low pressure (defining low torque capacity) and at high pressure (defining high torque capacity), the hydraulic pressure change arising from hysteresis is large in the low pressure region. Thus scatter arises in the transmitted torque, making it difficult to provide the desired torque with high precision. Therefore, even if feedback control is effected, for example, the deviation or error between the desired value (desired hydraulic pressure) and the controlled variable (hydraulic pressure supplied) is large and degrades vehicle riding comfort.
In particular, this problem is encountered when torque capacity has to be increased in response to increasing engine output. Higher torque capacity can be achieved either by expanding the hydraulic clutch pressure control range or by raising the clutch torque capacity, but whichever alternative is selected, the torque capacity control gain inevitably rises relative to solenoid load to exasperate the torque capacity scatter caused by torque load hysteresis.